The present invention relates to a macrocrystalline, laminar aluminum oxide, hereinafter called alumina or corundum, as well as to a method for manufacturing it from aluminum hydroxide (hydrargillite).
Macrocrystalline aluminum oxide is known and is presently obtained either from electrocorundum or from sinter alumina (tabular alumina). For the manufacture of lapping, grinding or polishing agents, calcined alumina or bauxite is initially fused in electrical resistance furnaces to produce electrocorundum which, after solidification, is brought to the desired grain size by breaking, grinding, sifting, possibly sorting and washing. Problems and difficulties in this process arise as a result of the hardness of the material to be ground and in setting the desired grain size and shape.
Tabular alumina is highly pure aluminum oxide and is largely produced by sintering calcined alumina at temperatures between 1800.degree. and 1950.degree. C. By comminution, particularly grinding of the sintered pellets, a mixture of monocrystals is obtained, and this mixture is then fractionated into the desired grain size ranges. Due to its good heat conductivity, temperature shock resistance and extraordinary mechanical hardness values, tabular alumina is especially suitable as a raw material for high quality fireproof products and other ceramic products.
The aluminum oxide required as a starting product for the above-described aluminas is obtained by the known calcination of aluminum hydroxide from the Bayer process in a rotary drum furnace or in a fluidized bed furnace at temperatures above 1200.degree. C.
Powdered aluminum oxide produced in this manner consists of agglomerates which are composed of a plurality of monocrystals, the particle size of the agglomerates lying between 10 and 300.mu.. When the agglomerates are ground, primary crystals are obtained whose average particle size, in dependence on the calcination method employed, is a maximum of 25.mu.. The primary crystals usually have a particle size of about 3 to about 12.mu..
According to the process disclosed in U.S. Pat. No. 4,130,402 to Schepers et al, primary crystals of aluminum oxide of an average particle size in particular, up to 16.mu., and a maximum average particle size up to about 25.mu., can be produced by the calcination of aluminum hydroxide in the presence of at least one fluorine salt and one vanadium salt. The term "vanadium salt" is understood to mean the foreign salts separated in many alumina factories from the concentrated sodium aluminate liquor, which contains, in addition to large quantities of sodium carbonate, fluctuating quantities of vanadium compounds, phosphorous compounds, fluorine compounds, and arsenic compounds. As is known, however, and as has been found again, the average particle size of the primary crystals cannot be increased by increasing the amounts of mineralizers present during calcination. Increases in temperature or the use of other or additional mineralizers either do not influence, or only insignificantly influence the average primary crystal size or the shape of the crystals.
Macrocrystalline alumina whose crystal faces have been formed by natural growth, for example, during the calcination according to U.S. Pat. No. 4,130,402 to Schepers et al, differs from alumina crystals obtained from fused corundum or from sinter alumina by means of mechanical forces, mainly in that the individual particles formed by natural growth have uniform crystal structure. The advantages resulting from uniform crystal structure become apparent when the crystals are used as a grinding agent and when they are employed to produce high quality ceramics.